Section 5: Velocity Protection and Control Devices

Excess Velocity

Several possible solutions are available for both protection
and control to minimize the negative effects of excessive velocity.
Solutions are categorized as either velocity protection devices
or velocity control devices.

Velocity Protection Devices

A velocity protection device does not necessarily reduce excessive
velocity but does protect threatened features from damage. Such
devices are usually economical and effective in that they serve
to provide a physical interim for the flow to return to a more natural
velocity. The protection devices discussed here include the following:

Channel liner
-- Most of the various types of channel liner have proven effective
for erosion protection. Some types of channel liner include low
quality concrete (lightly reinforced), rock, soil retention blankets,
articulated concrete blocks, and revetment mattresses. Channel liners, when
used as an outlet velocity protection measure, should be applied
to the channel area immediately downstream of the culvert outlet
for some distance, possibly to the right of way line and beyond
(with appropriate easement). (See Chapter 6 for types and guidelines.)
These liners, however, are viewed as creating environment problems,
including decreased habitat and increased water temperature. They
also are viewed to increase impervious cover, decrease time of concentration,
and change the hydrograph timing downstream. In many instances,
the liner may stabilize the area in question, only to have the problem
shift downstream to where the channel is not lined.

Pre-formed outlets - Pre-formed outlets
approximate a natural scour hole but protect the stream bed while
dissipating energy. These have been shown to be effective protection
in areas threatened by excessive outlet velocities. Such appurtenances
should be lined with some type of riprap. (A velocity appurtenance
for a culvert may be classified broadly as either a protection device
or a control device.)

Channel recovery reach -- Similar to a
pre-formed outlet, a channel recovery reach provides a means for
the flow to return to an equilibrium state with the natural, unconstricted
stream flow. The recovery reach should be well protected against
the threat of scour or other damage.

Velocity Control Devices

A velocity control device serves to effectively reduce an
excessive culvert outlet velocity to an acceptable level. The design
of some control devices is based analytically while, for others,
the specific control may be unpredictable. Some velocity control
devices are as follows:

Natural hydraulic jumps (most control
devices are intended to force a hydraulic jump) -- Most velocity
control devices rely on the establishment of a hydraulic jump. Because
a culvert being on a relatively steep slope usually results in excessive
outlet velocity from the culvert, the depth downstream of the culvert
exit is usually not great enough to induce a hydraulic jump. However,
some mechanisms may be available to provide a simulation of a greater
depth necessary to create a natural hydraulic jump.

Broken-back culvert configuration -- One
mechanism for creating a hydraulic jump is the broken back configuration,
two types of which are depicted in Figure 8‑26 and Figure 8‑27.
When used appropriately, a broken back culvert configuration can
influence and contain a hydraulic jump. However, there must be sufficient
tailwater, and there should be sufficient friction and length in
unit 3 (see Figure 8‑26 and Figure 8‑27) of the culvert. In ordinary
circumstances for broken back culverts, you may need to employ one
or more devices such as roughness baffles to create a high enough
tailwater.

Sills -- The use of the sill is effective
in forcing a hydraulic jump in culverts. One disadvantage of sills
is the possible susceptibility for silting. Sills must usually be
maintained frequently to keep it free of sediment deposition. Another
disadvantage is the waterfall effect that they usually cause. Riprap
should be installed immediately downstream of the sill for a minimum distance
of 10 feet to protect features from the turbulence of the waterfall
effect.

Roughness baffles
-- Roughness baffles, sometimes referred to as 'dragon's teeth',
can be effective in inducing turbulence, dissipating energy, and
reducing culvert outlet velocity (see Figure 8-25). Care must be
taken in the design and placement of the baffles; if the baffles
are too small or placed too far apart, they are ineffective. In
addition, they may interfere with mowing operations around the culvert
outlet. If these become damaged or broken from being hit by a mower,
they are ineffective. To limit the amount of potential damage, baffles
must be reinforced with rebar.

Energy dissipators -- An efficient but
usually expensive countermeasure is an energy dissipator. Some energy
dissipators have an analytical basis for design while others are
intended to cause turbulence in unpredictable ways. With turbulence
in flow, energy is dissipated and velocity can be reduced.

Other controls are described in the FHWA publication Hydraulic
Design of Energy Dissipators for Culverts and Channels, HEC-14.

Provided that unit 1 is on a mild slope,
its length has no effect on the outlet velocity of any downstream
hydraulic function. It is recommended that unit 1 either not be
used or be very short; the result is additional latitude for adjustment
in the profiles of units 2 and 3.

A longer unit 3 and a milder (but still
steep) slope in unit 2 together enhance the possibility of a hydraulic
jump within the culvert. However, these two conditions are contradictory
and usually not feasible for a given culvert location. Make some
compromise between the length of unit 3 and the slope of unit 2.
Unit 3 must be on a mild slope (du > dc).
This slope should be no greater than necessary to prevent ponding
of water in the unit. Do not use an adverse (negative) slope.

Starting at the upstream end of unit 2,
calculate a supercritical profile, beginning at critical depth and
working downstream through unit 3. The
Direct
Step Backwater Method is appropriate. Note the following:

Because uniform depth is now greater than
critical depth (mild slope), and flow depth is lower than critical
depth, the flow depth tends to increase towards critical depth.
Therefore, in unit 3, d should
be positive.

Calculate the elevation of sequent depth
(ds + flow line elevation) and compare it
with the tailwater elevation. Tailwater elevation may be a natural
stream flow elevation, or may produced artificially by installing
a sill on the downstream apron between wingwalls. Design Division
Hydraulics does not recommend the use of sills. (see
Velocity
Control Devices). If sills are used, the total vertical dimension
of the artificial tailwater is determined by adding the elevation
at the top of the sill and the critical depth of design discharge
flow over the sill. Base this critical depth on the rectangular
section formed by the top of the sill and the two vertical wingwalls.
If the elevation of sequent depth is lower than the tailwater elevation,
the following points apply; go to Step 5:

The broken-back culvert configuration
is ineffective as a velocity control device and should be changed
in some manner. Alternatives include rearrangement of the culvert profile,
addition of a sill, and investigation of another device. If the
profile is reconfigured, go back to step 3. Otherwise, skip step
5 and seek alternative measures.

The hydraulic jump may not occur within
the barrel under other flow conditions. It is wise to check the
sensitivity of the hydraulic jump to varying flow conditions to
help assess the risk of excessive velocities.

If a sill has been employed to force an
artificial tailwater, and the hydraulic jump has formed, the outlet
velocity calculated represents the velocity of water as it exits
the barrel. However, the velocity at which water re-enters the channel
is the crucial velocity. This velocity would be the critical velocity
of sill overflow.

Energy Dissipators

Stilling basins are hydraulically similar to sills (Figure
8-29). However, they are more expensive in construction and could
present serious silting problems. A chief advantage in stilling
basins is the lack of a waterfall effect.

Radial energy dissipators are quite effective but extremely
expensive to construct and, therefore, not ordinarily justified
(Figure 8-30). They function on the principle of a circular hydraulic
jump. For a detailed discussion on dissipator types, along with
a variety of design methods for velocity control devices, refer
to
HEC-14.